With this I will make a couple of assumptions:
a) All subdomains point to the same code in the file system (same document root)
b) All sites will operate under the same webapp.com domain (i.e. app1.webapp.com, app2.webapp.com, etc.)
Here is one possible way to accomplish what you are trying to do.
- - IIS, IP-, ( "All Unassigned" ). .
- DNS webapp.com, IP- -. , - , DNS, .
- SSL *.webapp.com. SSL- , . SSL , , - IIS.
- ( , , ..).
, WordPress.